Guardamos las mochilas en el maletero y dejamos una manta por si hacía frío.

They are in the preterite because they describe completed actions in the past:

• guardamos = we put away / we stored
• dejamos = we left

The sentence is telling a sequence of events: first we put the backpacks in the boot, then we left a blanket. In Spanish, the preterite is very common for this kind of completed narration.

Hacía frío uses the imperfect, which is natural here because it describes a background condition rather than a completed event.

In por si hacía frío, the idea is:

• in case it got cold / in case it was cold

Spanish often uses the imperfect after por si when talking about a possible situation someone was preparing for.

Compare:

• Dejamos una manta por si hacía frío. = We left a blanket in case it got cold.
• Hizo frío. = It got cold / It was cold.
  This would sound more like a specific fact that actually happened, not just a possibility being anticipated.

Por si means in case.

It introduces a possible situation that someone is preparing for:

• Dejamos una manta por si hacía frío.
• We left a blanket in case it got cold.

A very useful pattern is:

• por si + imperfect
• por si + present
• por si + clause

Examples:

• Lleva agua por si tienes sed. = Take water in case you get thirsty.
• Cogí el paraguas por si llovía. = I took the umbrella in case it rained.

Because por si is a fixed expression meaning in case. You cannot replace it with para si here.

• por si = in case
• para usually means for, in order to, or intended for

So:

• dejamos una manta por si hacía frío = correct
• dejamos una manta para si hacía frío = not correct

This is about how Spanish uses articles.

• las mochilas = the backpacks
• el maletero = the boot / trunk
• una manta = a blanket

Why definite articles with mochilas and maletero? Because the speaker is referring to specific, identifiable things in the situation: our/the backpacks and the boot of the car.

Why una manta? Because it introduces a blanket as an item, not necessarily one already known to the listener.

Spanish often uses the definite article where English might use a possessive:

• Guardamos las mochilas...
  English might say We put our backpacks...

Literally, guardar can mean to keep, to store, to put away, or to save, depending on context.

Here, guardamos las mochilas en el maletero means:

• we put the backpacks in the boot
• we stored the backpacks in the boot

It does not mean we kept the backpacks in the sense of we didn’t give them back. The phrase en el maletero makes the physical meaning clear.

Here dejamos means we left in the sense of we left behind / we left there / we placed and kept there.

So:

• dejamos una manta = we left a blanket

It does not mean we departed. In Spanish, dejar often means:

• to leave something somewhere
• to leave behind
• to let / allow

Context tells you which meaning is intended.

Spanish often prefers the definite article (el, la, los, las) instead of a possessive (mi, tu, nuestro) when ownership is obvious from context.

So:

• Guardamos las mochilas en el maletero
  naturally means We put the backpacks in the boot, often understood as our backpacks

Using nuestras mochilas is possible, but it puts more emphasis on ownership:

• Guardamos nuestras mochilas en el maletero

In many everyday situations, Spanish sounds more natural with las mochilas.

Yes, maletero is the standard word in Spain for the car boot.

For a learner used to other varieties of Spanish, this is useful:

• Spain: maletero
• In many Latin American countries, other words are common, such as baúl, cajuela, or maletera, depending on the country.

So if you are learning Spanish from Spain, maletero is exactly the word you want.

Because en el maletero tells you where the backpacks were put, and in Spanish it is very normal to place this kind of location phrase after the direct object:

• Guardamos las mochilas en el maletero.

This is the most natural, neutral order.

You could move elements around for emphasis, but the original sentence is the standard way to say it.

Yes, depending on context.

Literally, hacía frío is it was cold or it was cold weather. But in English, after in case, we often say:

• in case it got cold

So in this sentence, both ideas are close:

• We left a blanket in case it was cold
• We left a blanket in case it got cold

The second is often more natural in English, even though the Spanish uses the imperfect.

Because Spanish usually doesn’t need subject pronouns when the verb ending already shows who is doing the action.

• guardamos = we put away
• dejamos = we left

The ending -amos already tells you the subject is we.

You could say Nosotros guardamos..., but that would usually add emphasis or contrast:

• Nosotros guardamos las mochilas, no ellos.
  = We put the backpacks away, not them.

Yes. Y simply means and, and it connects two coordinated past actions:

1. Guardamos las mochilas en el maletero
2. dejamos una manta por si hacía frío

This is a very common way to narrate events in Spanish: a series of preterite verbs joined by y.

No, not with the same meaning.

• por si hacía frío = in case it was/got cold
• si hacía frío = if it was cold

That is an important difference:

• por si expresses precaution
• si expresses a condition

So the sentence means they left the blanket as a precaution, not only on the condition that it was cold.